This review examines the mechanism of translocation of cytoplasmically synthesized proteins into mitochondria. Approximately 10% of the mitochondrial proteins are synthesized within the organelles while most mitochondrial proteins are coded for by nuclear genes and synthesized on cytoplasmic ribosomes. Those mitochondrial proteins synthesized on cytoplasmic ribosomes have to be transferred at some point into one of the mitochondrial compartments, a process which would require their insertion through one or both mitochondrial membranes. Data accumulated during the past five years indicate that the cytoplasmically synthesized mitochondrial proteins are synthesized on free polysomes then released into the cytoplasm. Most of the proteins examined so far are synthesized in the cytoplasm as larger precursors whose conformations may differ from the conformations of their respective mature forms. These precursor proteins become translocated into mitochondrial post-translationally and processed to their mature forms either during or immediately following translocation into the organelles. The translocation step appears to require mitochondrial ATP. Some processing activities have been localized in the matrix fractions of mitochondria from liver and yeast and they appear to be associated with soluble endopeptidases which act selectively on precursors of mitochondrial proteins. Although it is not clear how the precursor proteins interact with or recognize mitochondrial membranes, studies in yeast indicate that the interactions occur at specific regions on the other mitochondrial membranes.

Recent characterization of brain and pituitary RNA metabolism is reviewed. Relative to other tissues, the brain transcribes more of the unique, single-copy DNA. This transcriptional diversity reflects the inherent heterogeneity in organization and development of the brain. The end product of transcriptional regulation in the brain is a population of functional cytoplasmic mRNAs with multiple components, differing in complexity and abundance. Analysis of nuclear and cytoplasmic RNA provides evidence that both brain-specific synthesis and processing may determine the final mRNA population. Both polyadenylated and non-polyadenylated RNA classes contribute significantly to the total brain polysomal mRNA fraction. Characterizations of individual species of mRNA from both brain and pituitary are described. One possible transcriptional modulator in both the pituitary and brain is the presence of steroid hormone at responsive sites. Functional consequences of steroid accumulation within the brain may be (1) interactions with neurotransmitter, especially catecholamine, metabolism and function, (2) developmental interactions with neuronal systems, and (3) differentiation of glial cell function. The pleiotropic nature of steroid hormone effects (both transcriptional and non-transcriptional) within one brain region is considered by examining the biochemical effects of glucocorticoids in the hippocampus.

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The rough endoplasmic reticulum isolated from several eukaryotic cell lines can be separated into subfractions. These subfractions possess different properties indicating that they represent separate domains of the endoplasmic reticulum system.